Electric wire for use at high temperatures



June 16, 1964 E. D. TEAGUE 3,137,766

ELECTRIC WIRE FOR USE AT HIGH TEMPERATURES Filed Dec. 12, 1958 FIG. I

F I 6. 2 F I 6. 3

PLATINUM 0R PLATINUM 0R PLATINUM ALLOY PLATINUM ALLOY PLATINUM 0R PLAT'NUM ALLOY w E PLATINUM ALLOY WIRE INVENTOR MOT-726W United States Patent ()fiice 3,137,766 ELECTRIC WIRE FOR USE AT HIGH TEMPERATURES rnest Douglas Teague, Welwyn Garden City, England,

asslgnor to Norton Company, Worcester, Mass, a corporation of Massachusetts Filed Dec. 12, 1958, Ser. No. 779,957 Claims priority, application Great Britain Dec. 16, 1957 3 Claims. (Cl. 174-126) The invention relates to electric wire of the platinum family, the wire being made of ruthenium, rhodium, palladium, osmium, iridium or platinum or alloys thereof. In most cases the major portion of the metal will be platinum. This application is based upon British complete specification No. 39,043 of December 16, 1957, now British Patent 856,309, December 14, 1960.

One object of the invention is to provide wire of the platinum family which volatilizes more slowly at high temperatures than has heretofore been the case. Another object of the invention is to provide wire of the platinum family that has less tendency to embrittlement and fracture. Another object is to slow down the crystal growth of wire of the platinum family. Another object is to produce superior electric wire for use as heating elements. Another object is to provide superior electric wire which is to be used at high temperature.

Other objects will be in part obvious or in part pointed out hereinafter.

In the drawings:

FIGURE 1 is a diagrammatic view of apparatus which can be used for toughening the wire, for flame spraying it with metal and for flame spraying it with oxide;

FIGURE 2 illustrates a wire having thereon a flame sprayed coating of metal; and

FIGURE 3 illustrates a wire having thereon a flame sprayed coating of metal and a second flame sprayed coating of metal oxide.

though the principal use now envisioned for the electrical wire of the present invention is for heaters, it can also be used in other apparatus such as thermocouples. In other words the high temperature may be produced by passing electric current through the wire or the wire may be heated in whole or in part in other manners and the electric current may be sent through the wire for other purposes. Although frequently round in cross section, the electric wire of the present invention can also be of ribbon shape or of any other desired shape in cross section. While usually the wire is long enough so that it can be wound, the invention also includes lengths which are short relative to the cross section so as to be in effect rods.

I have found that electric wire of the platinum family can be greatly beneficiated by flame spraying it with a coating of refractory oxide, especially by coating it with zirconia or alumina or a combination of the two. This coating can be applied as described in US. patent to W. M. Wheildon, In, No. 2,707,691 of May 3, 1955. In that patent Wheildon carefully explains what he means by stable metal oxide which is what he uses for coating. I can use any stable metal oxide as defined in the Wheildon patent.

I have further found that electric wire of the platinum family can be further beneficiated by first flame spray coating it with metal of the platinum family. This can be done by use of a flame spraying gun. Flame spraying guns to fuse, to atomize and to spray metal wire to produce a coating of metal are now very well known and can be bought from a number of manufacturers. The basic US. patent in this art is apparently No. 1,100,602 granted June 16, 1914, to Erika Morf of Zurich, Switzerland.

3,137,766 Patented June 16, 1964 There have been many other patents on flame spraying guns. The above mentioned patent to Wheildon discloses directly and by reference to patents including the Morf patent a flame spraying gun used to fuse, to atomize and to spray stable metal oxide.

Referring now to FIGURE 1 of the drawing, the platinum family metal 10 is originally wound on a spool 11. In order to conserve this expensive wire, it has attached to it a leader wire 12 and a trailer wire 13 of some inexpensive metal such as copper. By twisting at 14 the leader wire 12 is joined to the platinum wire 10 and by twisting at 15 the leader wire 10 is joined to the trailer wire 13. The leader wire 12 goes to a spool 11:: which can be identical with the spool 11. These spools can be made of wood. The free end of the leader wire 12 is secured as by means of a brad 16 to the spool 11a and the free end of the trailer wire 13 is secured as by means of a brad 17 to the spool 11.

A metal pin 21 extends through and journals the spool 11. It extends through the hubs 23 of spring arms 25 secured by screws 27 to a block 29 into which is press fitted a shaft 31 journalled in a two part bearing 33 secured to the frame of the apparatus in any suitable manner not shown. Secured to the outer end of the shaft 31 is a pulley 35 over which runs a belt 36.

The foregoing provides a let-off device for the platinum wire with a friction brake constituted by the hubs 23 and the springs 25 and by pushing out the pin 21 the spool 11 can be readily removed. The belt 36 is driven which rotates the spool 11 on the axis of the shaft 31 and the spool 11 also rotates on the axis of the pin 21 by the pull of the wire.

The spool 11a is a take up spool driven by a belt 38 running over a pulley portion 39a of the spool 11a which can be seen as the pulley portion 39 of the spool 11.

I provide two identical rolls 4t) and 41. These can be made of wood covered with soft rubber coverings 42 and 43 and they are rotatably mounted on shafts 44 and 45. The wire 10 goes over these rolls 49 and 41 in FIG- URE 8 fashion as shown. Anywhere from a few such as three or four to a dozen or more courses around the rolls and 41 can be used depending upon how much wire is to be coated, but it is well to have a half a dozen or more courses to avoid wasting platinum when this is sprayed upon the wire 10. Better production is achieved having a dozen or more courses of the wire 10 around the rolls 40 and 41 for the application of the oxide coating and for the sand blasting.

The rolls 40 and 41 are contained in a rectangular box 50, the back, the ends, the top and the bottom of which can be made to sheet metal and the front of which consists of rubber curtains 51 and 52 with an opening 53. In the top of the box is an exhaust pipe 55 for connection to exhausting apparatus to draw the exhaust gases out of the box 50 for protection of the operator. Slots 57 and 58 are provided for passage of the wire 10, 12 and 13. A simple traverse device 61 known in the textile arts is provided to feed the wire onto the spool 11a so that it will not bunch up in one spot and another one 62 is provided to feed the wire onto the roller 41. The traverse device 61 should reciprocate slowly a distance equal to the length of the barrels of the spools at about one half an inch a minute. The traverse device 62 should reciprocate over a distance of only about /4 inch at the same speed. This keeps the courses of the wire 10 close together. These can be actuated by a suitable mechanism not shown.

In order not to crack the oxide coating, the roll 40 and 41 should be six inches in diameter or more and the spools 11 and 11a (since they are interchangeable) of as much diameter at the barrels. This completes the diagrammatic description of suitable apparatus which they can be -four inc-hes wide.

can be used to make the electric wire of the invention and of course it is illustrative merely as many other schemes can be used, but this type of apparatus is the bestone now known to me. Good feed for the wire is four inches a minute and at that rate the shaft 31 should rotate, slowly, about one rotation in everyfour minutes which'is my best estimate at present.

The apparatus shown in FIG. 1 is first used for roughening the platinum wire 10. This is done by socalled sand blasting. There are other ways, but-it has been found that sand blasting'is the best. This is a general term and while in the beginning of the sand blasting art, sand was used, for better results fine aluminum oxide grit is preferred. It is. difficult to produce a roughened surface upon wire of less than four thousandths of an inch in diameter and illustratively the wire may be ten thousandths of an inch in diameter. In such a case fused aluminum oxide grit of 180 grit size is used. This fine grit has been found to be about the best for roughening wires, coarser grit being used for steel plates when sand blasting. There is available grit of much finer size than 180 mesh but it does not seem to make such a rough surface even on wires as does 180 grit which is the same thing as 180 mesh.

The same apparatus is then used for coating the roughened platinum wire'. Preferably the wire is first coated with a metal such as platinum or platinum base alloy as shown in FIGURE 2. The wire may then be coated with metal oxide as shown in FIG. 3.

Example I Platinum wire of 10 mils diameter is passed through the machine and sand blasted as described. The sand blasting stream is directed to the place where the wires cross. Here is found a crossing sheet of wires, the rollers 40 being wide enough to receive a sheet, for example Sand blasting apparatus is well known and need not be described. So also is the technique of sand blasting. After all of the wire 10 has passed in front of the sand blast nozzle which is at or near the opening 53, all parts of the circumference and lengthof the wire 10 will be roughened.

Now, using the same apparatus, a coating of alumina is flame-sprayed in accordance with the Wheildon patent onto the platinum family metal 10. This coating of alumina adheres strongly to theplatinum wire and covers the whole length all around the circumference. Proceeding with the same feed as above stated, the coating will be about .002 inch.

Example 11 Platinum wire is coated with fused zirconium oxide by the method of the Wheildon patent first sand blasting as herein described and all procedure is the same except that instead of using an aluminum oxide rod in the flame spray gun, a zirconium oxide rod is used. As will be seen from the Wheildon patent, the atomized spray of fused metal oxide is produced by fusing, atomizing and spraying metal oxide rods.

Example III This is just like Example I only prior to flame spraying the wire 10 with aluminum oxide, it is flame sprayed with platinum family metal with a metal spraying gun such as described in the Morf patent. For spraying platinum as well as spraying the oxides, a flame of acetylene burning in oxygen is preferably used such as described in the Wheildon patent and this statement applies to Examples I and II, namely that preferably the flame is produced by acetylene and oxygen. This has a temperature of combustion suflicient to fuse the zirconia which melts at just below 2700 C. Aluminum oxide melts at just above 2000 C. while platinum melts at about 1773 C. and the other metals of the platinum family all melt below 2700 C.

4;- Example IV This is the same as Example III only the second coating is a coating of zirconium oxide instead of aluminum oxide.

For many practical purposes a platinum wire .010" in diameter is good. A practical :coating of oxide thereon is .002" in thickness and a practical coating of platinum thereon is .002" in thickness. But it is impossible to state the best mode of the invention since the requirements for electric heating wire are multitudinous. Still I can say this to comply with the new statutory requirement for the best mode: the wire is round, it is platinum meaning as the metal comes, containing other metals of the family which are found in native platinum less than 5%. The metal for coating is the same platinum. The wire (and of course this depends on requirements) is .015" in diameter, a coating of platinum .003" is applied and then a coating of aluminum oxide .003" is applied. This is the best I can do to define the best mode. For thicker wires thicker coatings are preferred. Wires thinner than .004" are with difliculty coated at all as they are diflicult to roughen by sand blasting and other roughening methods are not so good.

Electric wire as described herein is superior to plain platinum wire (meaning platinum family metal of any kind including what is called platinum alloy which means that other metal is deliberately added) because it lasts longer. Under identical conditions of test, an ordinary platinum strip resistor showed a 12 /2 loss by volatilization and a large crystal development, while the platinum strip which was coated with flame sprayed platinum and then coated with flame sprayed alumina showed no loss by volatilization and no apparent crystal development. The heat testing temperature in each case was the same, the length of the test was the same, and the atmosphere was air. The cross section of the platinum strip in each case was inch x inch. The coating of sprayed platinum was 0.002 inch and the coating of sprayed alumina was 0.010 inch. Alumina and aluminum oxide herein are synonymous. Large crystal development results in early failure of electric strip resistor operated at high temperatures because it leads to embrittlement and fracture of the metal. I did not test a platinum strip coated only with oxide, but I did test a platinum strip of the same cross section coated with the same thickness of sprayed platinum and this showed a 7% loss by volatilization and no apparent crystal developement. However by the tests made, it is clear that a platinum family wire or strip coated only with stable oxide is superior to one not coated at all.

The undercoating of platinum family metal flame sprayed in situ under the coating of oxide flame sprayed in situ is also preferred because the difference in coeflicient of expansion of the oxide and of the platinum is compensated by the resilience of the flame sprayed platinum. Flame sprayed coatings of both kinds are porous and will stretch and compact more readily than solid metal or solid oxide. Therefore from another point of view, the best embodiment of the invention is that in which the wire is coated both with platinum family metal and with stable oxide. On the other hand, platinum is expensive and for some applications a single coating of stable oxide flame sprayed in situ is suflicient as it is not expensive to apply and the electric wire is beneficiated thereby.

Since these coatings are porous, it is remarkable that such improved results are achieved by applying them. While I have not made suflicient tests, it appears that the life of platinum electric wire is increased many fold by coating it according to the preferred embodiment of the invention. Indications are that the best oxide to use is alumina and this is also surprising because the Wheildon patent teaches that thisbecomes gamma alumina which is an unstable form of alumina. The stable form of alumina is alpha alumina. I have no explanation for the surprising result of the stopping of crystal development by the application of the two coatings. This appears to be an unpredictable advantage of the invention.

The wire of the invention consists of two parts or three parts. There is an inside wire of metal of the platinum family which is dense. It can be solid or hollow but it is dense metal such as made by drawing ingots. On this is a coating of stable refractory oxide flame sprayed in situ. In the best embodiment of the invention, on the dense inside wire is a coating of metal of the platinum family (e.g. pure platinum or commercial platinum or another metal of the platinum family or alloy of metals of the platinum family, and this goes also for the inside wire) flame sprayed in situ outside of which is a coating of stable refractory oxide flame sprayed in situ. It is impossible to describe these coatings except by saying that they are flame sprayed in situ. They can be made by flame spraying metal and oxide powders as well as by flame spraying wires and rods as described.

By the sand blasting a roughened surface is produced and any other way of producing a roughened surface can be used but sand blasting is best. If the wire is not too fine, it can be roughened by knurling. The refractory stable metal oxide interlocks with the roughened surface of the dense metal and produces a coating integral therewith. So also does the coating of metal of the platinum family flame sprayed in situ. In both forms of the invention all parts of the total wire are integral. The surface of the coating of metal of the platinum family that is flame sprayed in situ is rough by nature. It is porous and undercut and presents excellent anchorage for the supercoating of refractory stable metal oxide that is flame sprayed in situ, and this well interlocks with it as it is also porous and has rough surfaces.

It will thus be seen that there has been provided by this invention an electric wire for use at high temperatures in accordance with which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiment above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. Electric wire comprising an inside wire of dense metal selected from the group consisting of platinum and platinum base alloy, said wire having a roughened surface, an integral coating thereon of metal selected from the group consisting of platinum and platinum base alloy, said metal flame sprayed in situ and interlocked with the roughened surface of the dense metal, and a further rigid porous integral coating of interlocking laminate structure of refractory stable metal oxide flame sprayed in situ and interlocked with the surface of the flame sprayed metal coating, all thereof being integral with each other.

2. Electric wire according to claim 1 in which the coating of refractory stable metal oxide is gamma alumina.

3. Electric wire according to claim 1 in which the coating of refractory stable metal oxide is zirconia.

References Cited in the file of this patent UNITED STATES PATENTS 1,001,105 Welsbach Aug. 22, 1911 1,422,443 Hoge July 11, 1922 2,041,286 Erber et al May 19, 1936 2,060,081 l'edele Nov. 10, 1936 2,065,988 Sperti Dec. 29, 1936 2,106,527 Hostetter Jan. 25, 1938 2,319,364 Ziegs May 18, 1943 2,320,327 Meduna May 25, 1943 2,375,154 Volterra May 1, 1945 2,470,051 Ruge May 10, 1949 2,707,691 Wheildon May 3, 1955 2,805,965 Robinson Sept. 10, 1957 2,819,207 Shepard Jan. 10, 1958 2,868,667 Bowles Jan. 13, 1959 2,872,352 Porter et a1. Feb. 3, 1959 2,885,334 Green May 3, 1959 2,973,283 Hill Feb. 28, 1961 FOREIGN PATENTS 23,098 Great Britain Dec. 21, 1905 490,116 Great Britain Aug. 9, 1938 

1. ELECTRIC WIRE COMPRISING AN INSIDE WIRE OF DENSE METAL SELECTED FROM THE GROUP CONSISTING OF PLATINUM AND PLATINUM BASE ALLOY, SAID WIRE HAVING A ROUGHENED SURFACE, AN INTEGRAL COATING THEREON OF METAL SELECTED FROM THE GROUP CONSISTING OF PLATINUM AND PLATINUM BASE ALLOY, SAID METAL FLAME SPRAYED IN SITU AND INTERLOCKED WITH THE ROUGHENED SURFACE OF THE DENSE METAL, AND A FUTHER RIGID POROUS INTEGRAL COATING OF INTERLOCKING LAMINATE STRUCTURE OF REFRACTORY STABLE METAL OXIDE FLAME SPRAYED IN SITU AND INTERLOCKED WITH THE SURFACE OF THE FLAME SPRAYED METAL COATING, ALL THEREOF BEING INTEGRAL WITH EACH OTHER. 